Abstract
We developed a compact, low-cost and simple hydraulic hybrid system. This system has regeneration state, assist state and engine start-stop function state. These states are controlled by an oil circuit with spool valve and electric linear solenoid. We developed two important components, a gear pump/motor that works as both pump and motor with high efficiency and a low-flow force spool valve with pressure control groove. We applied this technology to a small car and achieved smooth regeneration/assist, low noise and quick engine restart by cooperative engine control. This system improves vehicle fuel economy 10.1% in Japanese JC08 mode.